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At least 55 records · Page 3Linked to original sources
USGS1988

Fossil diatoms and neogene paleolimnology

Diatoms have played an important role in the development of Neogene continental biostratigraphy and paleolimnology since the mid-19th Century. The history of progress in Quaternary diatom biostratigraphy has developed as a result of improved coring techniques that enable sampling sediments beneath existing lakes coupled with improved chronological control (including radiometric dating and varve enumeration), improved statistical treatment of fossil diatom assemblages (from qualitative description to influx calculations of diatom numbers or volumes), and improved ecological information about analogous living diatom associations. The last factor, diatom ecology, is the most critical in many ways, but progresses slowly. Fortunately, statistical comparison of modern diatom assemblages and insightful studies of the nutrient requirements of some common freshwater species are enabling diatom paleolimnologists to make more detailed interpretations of the Quaternary record than had been possible earlier, and progress in the field of diatom biology and ecology will continue to refine paleolimnological studies. The greater age and geologic setting of Tertiary diatomaceous deposits has prompted their study in the contexts of geologic history, biochronology and evolution. The distribution of diatoms of marine affinities in continental deposits has given geologists insights about tectonism and sea-level change, and the distribution of distinctive (extinct?) diatoms has found utilization both in making stratigraphic correlations between outcrops of diatomaceous deposits and in various types of biochronological studies that involve dating deposits in different areas. A continental diatom biochronologic scheme will rely upon evolution, such as the appearance of new genera within a family, in combination with regional environmental changes that are responsible for the wide distribution of distinctive diatom species. The increased use of the scanning electron microscope for the detailed descriptions of fossil diatoms will provide the basis for making more accurate correlations and identifications, and the micromorphological detail for speculations about evolutionary relationships.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1988

An isotopic study of a fluvial-lacustrine sequence: The Plio-Pleistocene koobi fora sequence, East Africa

Stable isotopic analyses of Plio-Pleistocene and modern sediments in the fluvial-lacustrine system occupying the Turkana Basin, East Africa provide constraints on the paleoenvironmental and diagenetic histories of the Pliocene through the Recent sediments in the basin. The δ 13 C values for carbonates in lacustrine sediments range from −15 to +22‰ relative to PDB, depending on the varying proportions of CO 2 from the atmospheric reservoir and from various metabolic sources. The δ 18 O values of carbonates in lacustrine sediments indicate that the isotopic composition of paleolake water varied by over 10‰ from the Pliocene to the present. The δ 13 C values for pedogenic carbonates record paleoccologic variations and suggest that C 4 plants did not become well established in the preserved depositional parts of the basin until about 1.8 myr ago. The δ 18 O values pedogenic carbonates suggest a range of over 10‰ for the isotopic composition of soil water during this interval. They also suggest a period of major climatic instability from about 3.4 to 3.1 myr and at about 1.8 myr. Together, the δ 13 C and δ 18 O values of pedogenic carbonates indicate that the present conditions are as arid and hot as any that had prevailed during deposition of these Plio-Pleistocene sediments.

Lake Turkana Basin↗
USGS1989

The potential of paleozoic nonmarine trace fossils for paleoecological interpretations

Many Late Paleozoic environments have been interpreted as marine because of the co-occurrence of supposedly exclusively marine trace fossils. Beginning in the Late Ordovician, however, nonmarine trace-fossil diversity increased throughout the Paleozoic. This diversification of nonmarine organisms and nonmarine trace fossils was especially prevalent in Devonian and later times. Diversification of freshwater organisms is indicated by the large number of freshwater fish, arthropods, annelids and molluscs that had developed by the Carboniferous. In addition to diverse freshwater assemblages, entirely terrestrial vertebrate and invertebrate ecosystems had developed by the Devonian. This rapid diversification of freshwater and terrestrial organisms is inherently linked to development and diversification of land plants and subsequent shedding of large quantities of organic detritus in nonmarine and marginal-marine areas. Nearshore marine organisms and their larvae that are able to tolerate relatively short periods of lowered salinities will follow salt-water wedges inland during times of reduced freshwater discharge. Similarly, amphidromous marine organisms will migrate periodically inland into nonmarine environments. Undoubtedly, both of these processes were active in the Paleozoic. However, both processes are restricted to stream/distributary channels, interdistributary bays, or estuaries. Therefore, the presence of diverse trace-fossil assemblages in association with floodplain deposits is interpreted to reflect true nonmarine adaptation and diversity. Conversely, diverse trace-fossil assemblages in association with stream/distributary channel deposits, interdistributary-bay deposits, or estuarine deposits may reflect migration of salt-water wedges inland, or migration of marine organisms into freshwater environments (amphidromy), or both.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1989

Dinoflagellate species and organic facies evidence of marine transgression and regression in the atlantic coastal plain

Palynological evidence is used to date and interpret depositional environments of sediments of Campanian, Maestrichtian and early Danian ages cored in three wells from South Carolina and Georgia. The evidence is usefil for distinguishing environments which lithofacies evidence indicates a range from nonmarine to coastal to inner neritic shallow shelf. Numerous dinoflagellate species and an organic facies defined abundant amoprphous debris (amorphous debris facies) distinguish shallow shelf sediments deposited during marine transgression. The nearshore amorphous debris facies of late Campanian age consists of heterogenous assemblages dominated by Palaeohystrichophora infusorioides Deflandre or Hystrichosphaerina varians (May). The farther offshore amorphous debris facies of late early Maestrichtian to late Maestrichtian age consists of heterogenous assemblages dominated by Glaphyrocysta retiintexta (Cookson) and/or Areoligera medusettiformis (Wetzel). The larger number of dinoflagellate species in the offshore facies represents the maximum transgression detected in the investigated interval. A multiple occurrence datum defined by the combination of first appearance, klast appearances and sole occurrence of dinoflagellate species at the base of each interval distinguished by the amorphous debris facies provides the first evidence of marine transgression. Relatively small organic residues consisting of intertinite and few or no palynomorphs define the inertinite facies in nonmarine deltaic and in coastal (lagoonal, tidal flat, interdistributary bary) sediments. Dinocyt{star, open}s are absent in the nonmarine sediments and are represented by few species and few specimens in the coastal inertinite faceis. A third organic facies (vascular tissue facies) is defined by the abundance of land plant tissue. Sporomorph species, including those of the Normapolles pollen group and of pteridophyte spores, comprise a large proportion of the total palynomorph flora in the inertinite and vascular tissue facies. The vascular tissue facies occurs in the proximal prodelta and nearshore shallow shelf lithofacies of early Maestrichtian age. Baed in the sequence of organuic facies, dinoflagellate species abundance, and lithofacies in the investigated wells, the nonmarine and coastal inertinite facies was first deposited and was followed during the late Campanian by a marine trangression when the nearshore amorphous debris facies was deposited. This was followed in the early Maestrichtian by the influx of terrigenous organic matter (vascular tissue facies) in response to deltaic progradation on the shallow shelf. A marine regression occurred towards the close of the early Maestrichtian, emplacing the coastal inertinite facies. The major marine transgression occurred near the end of the early Maestrichtian, developing a farther offshore amorphous debris facies on an expanded continental shelf which persisted through the late Maestrichtian. The inertinite facies returned during marine regression in the approximate position of the Maestrichtian/Danian boudnary. ?? 1989.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1989

Late cretaceous pelagic sediments, volcanic ASH and biotas from near the Louisville hotspot, Pacific Plate, paleolatitude ∼42°S

Dredging on the deep inner slope of the Tonga Trench, immediately north of the intersection between the Louisville Ridge hotspot chain and the trench, recovered some Late Cretaceous (Maestrichtian) slightly tuffaceous pelagic sediments. They are inferred to have been scraped off a recently subducted Late Cretaceous guyot of the Louisville chain. In the vicinity of the Louisville hotspot (present location 50°26′S, 139°09′W; Late Cretaceous location ∼42°S, longitude unknown) Late Cretaceous rich diatom, radiolarian, silicoflagellate, foraminiferal and coccolith biotas, accumulated on the flanks of the guyot and are described in this paper. Rich sponge faunas are not described. ? Inoceramus prisms are present. Volcanic ash is of within-plate alkalic character. Isotope ratios in bulk carbonate δ 18 O − 2.63 to + 0.85, δ 13 C + 2.98 to 3.83) are normal for Pacific Maestrichtian sediments. The local CCD may have been shallower than the regional CCD, because of high organic productivity. In some samples Late Cretaceous materials have been mixed with Neogene materials. Mixing may have taken place on the flanks of the guyot during transit across the western Pacific, or on the trench slope during or after subduction and offscraping about 0.5 Ma.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1989

Stratigraphy and depositional history of the Pliocene Bianco section, Calabria, southern Italy

An integrated micropaleontological and geochemical study was carried out on the Pliocene-age Bianco section located in Calabria, southern Italy. This section is somewhat unique for the Pliocene of the Mediterranean region in that it contains abundant calcareous and siliceous microfossils. Based on the biostratigraphic findings, it ranges in age from approximately 3.7-3.0 Ma. The Bianco section is composed of marly mudstones intercalated with diatomites, with the diatomites being particularly common in the upper 50 m of the section (above 3.1 Ma). The diatomites contain an abundant benthic foraminiferal assemblage and have a low organic carbon content indicating that bottom waters were fairly well-oxygenated during their deposition. Faunal and floral indicators suggest a cooling of surface waters in this region at 3.1 Ma. The diatom assemblages within the Bianco diatomites are very similar to those living in the Gulf of California, suggesting an upwelling origin for these silica-rich units. A model is proposed which attributes diatomite formation to upwelling induced by climatically controlled changes in local hydrography.

Pliocene Bianco section, Calabria↗
USGS1989

Tulelake, California: The last 3 million years

The Tulelake basin, formed by east-west extension and faulting during the past several million years, contains at least 550 m of lacustrine sediment. Interdisciplinary studies of a 334 m-long cored section from the town of Tulelake, California, near the center of the basin, document a 3-m.y. record of environmental changes. The core consists of a thick sequence of diatomaceous clayey, silty, and marly lacustrine sediments interbedded with numerous tephra layers. Paleomagnetic study puts the base of the core at about 3.0 Ma. Twelve widespread silicic tephra units provide correlations with other areas and complement age control provided by magnetostratigraphy; mafic and silicic tephra units erupted from local sources are also common in the core. Widespread tephra units include the Llao Rock pumice (=Tsoyawata, 7 ka), the Trego Hot Springs Bed (23 ka), and the Rockland (0.40 Ma), Lava Creek (0.62 Ma), and Rio Dell (1.5 Ma) ash beds, as well as several ash beds also found at Summer Lake, Oregon, and an ash bed originally recognized in DSDP hole 173 in the northeastern Pacific. Several tephra layers found in the core also occur in lacustrine beds exposed around the margins of the basin and elsewhere in the ancestral lacustrine system. Diatoms are present throughout the section. Pollen is present in most of the section, but some barren zones are found in the interval between 50 and 140 m; the greatest change in behavior of the pollen record takes place just above the top of the Olduvai Normal-Polarity Subchronozone. Ostracodes are present only in high-carbonate (>10% CaCO 3 ) intervals. Evolutionary changes are found in the diatom and ostracode records. Bulk geochemical analyses show significant changes in elemental composition of the sediment through time.

California↗
USGS1989

Stetson Pit, Dare County, North Carolina: An integrated chronologic, faunal, and floral record of subsurface coastal quaternary sediments

Continuous split spoon samples from a drill hole penetrating 34 m of coastal plain sediments at Stetson Pit in Dare County, North Carolina were taken for lithologic, aminostratigraphic, faunal (ostracodes) and floral (pollen) analyses. Three distinct aminozones are recognized in the subsurface section based upon D-alloisoleucine/L-isoleucine (A/I) values in each of the molluscan species Mulinia lateralis and Mercenaria sp. Ostracode zonations in the subsurface section are based on percentages of 80 thermophilic and cryophilic species (those living today south and north of Cape Hatteras) and the percentages of brackish water species. Five assemblage zones are delineated. Six pollen assemblage zones are also delineated within the subsurface section based upon study of 48 sediment samples. The subsurface record at Stetson Pit is interpreted to represent portions of four interglacials based upon the combined faunal, floral and aminostratigraphic data. The two younger aminozones, with amino acid age estimates of 100,000±20,000 yr (−7.2 to −11.2 m MSL) and 300,000–500,000 yr (−13 to −14.2 m MSL), represent portions of middle/late Pleistocene interglacials. The lower aminozone (−17.4 to −33 m MSL) spans an interval that probably includes at least two interglacials (based upon faunal and floral records) and has an age estimated to be between 800,000 and 1,300,000 yr. Boundaries delineated by faunal, floral, and amino acid methods do not always coincide, due to sampling constraints and phase lags between the different records. One major unconformity (at −17.4 m MSL) in the Stetson Pit section is easily recognized from lithologic characteristics and may represent a hiatus of as much as 800,000 yr. Lithologic changes associated with all other zone boundaries are subtle and would probably not be considered significant in the absence of faunal, floral, or amino acid data.

North Carolina↗
USGS1989

Geochemical and paleoenvironmental variations across the Cretaceous/Tertiary boundary at Braggs, Alabama

The Cretaceous/Tertiary (K/T) boundary in southern Alabama occurs in a sequence of interbedded shallow-marine limestones and marls deposited during a Late Maastrichtian regression and subsequent Danian transgression. The presence of a diverse assemblage of Cretaceous and Paleocene benthic micro- and macrofossils has allowed detailed examination of paleoenvironmental changes in this shallow-marine setting at the time of the K/T boundary extinctions. Although extensive diagenesis has resulted in the recrystallization and cementation of whole rock carbonate, a few molluscan macrofossils have retained their original calcitic structure. The oxygen isotopic record of these macrofossils shows a gradual cooling of 2–3°C over a 3 m.y. period which began in the late Maastrichtian and continued into the Danian. The characteristic depletion in δ 13 C across the K/T boundary displayed by planktonic microfossils from pelagic sequences is not recorded at Braggs. Major changes in whole rock trace and minor element chemistry reflect the decrease in terrigenous mineral fluxes to this location brought about by the rapid Early Paleocene transgression.

Alabama↗
USGS1990

Neogene paleoceanographic events recorded in an active-margin setting: Humboldt basin, California

Recognition of North Pacific paleoceanographic events in the marginal Humboldt (Eel River) basin of northern California enables correlation of stratigraphic sections and development of a chronostratigraphy. Paleoclimatically related coiling shifts in Neogloboquadrina pachyderma (Ehrenberg) and benthic foraminiferal datums form the basis of the chronostratigraphy. Benthic foraminiferal datums are defined by the occurrence of selected benthic species and abundance maxima of benthic biofacies. The compiled chronostratigraphy is used to refine reconstructions of the depositional history of Humboldt basin. Paleoceanographic events, recognized by the distribution of benthic foraminiferal biofacies, are used to infer paleoceanographic history along the northeastern Pacific margin. The similarity in coiling curves of N. pachyderma from the marine sequence at DSDP Site 173 and the coastal Centerville Beach section of Humboldt basin and at other independently dated sites along the northeastern Pacific margin demonstrates that matching records of climatic oscillations is a reliable method of correlating marine sequences. Benthic fauna from the Centerville Beach section vary in phase with climatically related coiling shifts in N. pachyderma . In particular these data show an increase in displaced neritic fauna during inferred warm intervals and resurgence of deeper bathyal fauna during inferred cool events. Similar data are observed from the inland Eel River section, demonstrating that benthic foraminiferal trends recognized at Centerville Beach can be identified elsewhere in Humboldt basin. This in-phase benthic response to climatic fluctuations probably results from changes in vertical depth range of many benthic species in response to paleoclimatically related vertical changes in water-mass position. Depositional histories reconstructed for two key sites in southern Humboldt basin indicate low rates of sediment accumulation during early basin filling with hemipelagic sediments. Initiation of turbidite sedimentation in the early Pliocene resulted in a sharp increase in rate of sediment accumulation. This increase in rate of sediment accumulation is partially a response to tectonic uplift in the northern Coast Ranges and may be an effect of realignment of motion between the Pacific and North American plates at about this time. The inland site shoaled more rapidly during turbidite sedimentation as a result of a higher rate of sediment accumulation. The rate of sediment accumulation increased again at this site in the late Pliocene during deposition of shelf and nearshore facies. The Eel River region subsided concurrent with deposition of these shallow-water deposits.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1990

Chronology of expansion and contraction of four great Basin lake systems during the past 35,000 years

During the past 35,000 years, Lake Bonneville, Lake Russell, and Lake Searles underwent a major period of lake-level change. The lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15,000 and 13,500 yr B.P. The rise of Lake Lahontan was gradual but not continuous, in part because of topographic constraints (intrabasin spill). Lake Lahontan also had an oscillation in lake level at 15,500 yr B.P. Radiocarbon-age estimations for materials that were deposited in the lake basins indicate that Lake Bonneville rose more or less gradually from 32,000 yr B.P., and had major oscillations in level between 23,000 and 21,000 yr B.P. and between 15,250 and 14,500 yr B.P. Lake Russell and Lake Searles had several major oscillations in lake level between 35,000 and 14,000 yr B.P. The timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states. All four lakes may have had nearly synchronous recessions between about 14,000 and 13,500 yr B.P. After the recessions, the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11,500 and 10,000 yr B.P. These data provide circumstantial evidence that the Younger Dryas Event affected climate on at least a hemispheric scale. During the Holocene, the four lakes remained at low levels, and small oscillations in lake level occurred. An important aspect of the lake-level data is the accompanying expansion of lake-surface area at the time of the last highstand. Lake Bonneville and Lake Lahontan had surface areas about 10 times larger than their mean-historical reconstructed areas whereas Lake Russell and Lake Searles had surface areas about 5 times larger than their mean-historical reconstructed areas. Differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream, or they may have resulted from lake/atmosphere feedback processes.

California, Idaho, Nevada, Oregon↗
USGS1990

Expression of seasonal and ENSO forcing in climatic variability at lower than ENSO frequencies: Evidence from Pleistocene marine varves off California

Upper Pleistocene marine sediments along the upper continental slope off northern and central California contain alternations of varved and bioturbated sediments and associated changes in biota and sediment composition. These alternations can be related to conditions that accompany El Niño and anti-El Niño (ENSO) circulation. Anti-El Niño conditions are characterized by increased upwelling and productivity and by low concentrations of dissolved oxygen in the oxygen minimum zone that resulted in varve preservation. El Niño conditions are characterized by little or no upwelling, low productivity, and higher concentrations of dissolved oxygen that resulted in zones of bioturbation. Alternations of varves and zones of bioturbation, that range from decades to millennia, occur through the upper Pleistocene section. The inferred long-term alternations in El Niño and anti-El Niño conditions appear to be a re-expression of ENSO's primary 3–7 year cycle. Decadal to millennial cycles of productivity associated with El Niño and anti-El Niño conditions may have served as a “carbon pump” and transferred atmospheric CO 2 to the marine reservoir. Changes in sediment composition and organisms associated with El Niño or anti-El Niño conditions can be related to both seasonal and ENSO phenomena. Expression of these changes at lower-than-ENSO frequencies may be partly explained by adding the effects of seasonal variability to effects produced by a self-oscillating ENSO system. However, deterministic mechanisms, including solar modulation of ENSO, may also contribute to long-term alternations of El Niño and anti-El Niño conditions.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1990

Accelerator-mass spectrometer (AMS) radiocarbon dating of Pleistocene lake sediments in the Great Basin

Pleistocene lake sediments in the Great Basin typically contain little organic carbon, and thus are difficult to date reliably by conventional radioccarbon methods. Paleoenvironmental data are abundant in these sediments, but are of limited value without adequate age controls. With the advent of accelerator-mass spectrometer (AMS) radiocarbon dating, it is now possible to date these paleolacustrine sediments. AMS dates were obtained on sediment cores from the Bonneville, Franklin, and Lahontan Basins. In the Bonneville Basin, the AMS-based chronology compares well with other chronologies constructed from dated shore-zone features. In the Bonneville and Franklin basins, AMS dates delimit unconformities not apparent by other means. We found that dispersed organic carbon from sediments deposited during relatively freshwater intervals provided apparently reliable AMS radiocarbon dates. Carbonate microfossils from the Lahontan Basin also produced results that appear reasonable, while bulk carbonate yielded erroneous results.

California, Nevada, Oregon, Utah↗
USGS1991

Dating lacustrine episodes in the eastern Sahara by the epimerization of isoleucine in ostrich eggshells

The eggshell of the African ostrich, Struthio camelus , closely approximates a closed system for the retention of indigenous proteinaceous residues. Epimerization of the protein amino acid isoleucine follows linear first-order kinetics in laboratory simulations nearly to racemic equilibrium, and the variation in D/L ratio within a single fragment, or between fragments of the same age, is significantly less than in other carbonate systems. These observations suggest that the extent of isoleucine epimerization (aIle/Ile ratio) in ostrich eggshell offers the potential for high-resolution geochronology of Quaternary deposits. From the simulation experiments, and dated early Holocene samples for which we have in situ mean annual sediment temperature measurements, Arrhenius parameters have been calculated; the activation energy is 30.33 kcal mol −1 , similar to that of other carbonate systems. We have measured the aIle/Ile ratio in ostrich eggshell associated with lacustrine episodes at Bir Tarfawi and Bir Sahara East, two depressions in what is currently the hyperarid eastern Sahara. The ratios can be used directly to indicate qualitatively the time represented by each series of lake sediment, and to correlate disjunct lacustrine deposits within and between the basins. Uranium-series disequilibrium dating of algal mats contained within some of the lake beds indicate that a major wet interval occurred about 130 ka ago. Using the U-series date for calibration, the amino acid ratios are used to date the most recent lacustrine interval to about 100 ka B.P., and two older intervals, one about 200 ± 25 ka B.P., and an older interval that occurred prior to 250 ka ago.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1991

Sedimentary facies and depositional environments of early Mesozoic Newark Supergroup basins, eastern North America

The early Mesozoic Newark Supergroup consists of continental sedimentary rocks and basalt flows that occupy a NE-trending belt of elongate basins exposed in eastern North America. The basins were filled over a period of 30–40 m.y. spanning the Late Triassic to Early Jurassic, prior to the opening of the north Atlantic Ocean. The sedimentary rocks are here divided into four principal lithofacies. The alluvial-fan facies includes deposits dominated by: (1) debris flows; (2) shallow braided streams; (3) deeper braided streams (with trough crossbeds); or (4) intense bioturbation or hyperconcentrated flows (tabular, unstratified muddy sandstone). The fluvial facies include deposits of: (1) shallow, ephemeral braided streams; (2) deeper, flashflooding, braided streams (with poor sorting and crossbeds); (3) perennial braided rivers; (4) meandering rivers; (5) meandering streams (with high suspended loads); (6) overbank areas or local flood-plain lakes; or (7) local streams and/or colluvium. The lacustrine facies includes deposits of: (1) deep perennial lakes; (2) shallow perennial lakes; (3) shallow ephemeral lakes; (4) playa dry mudflats; (5) salt-encrusted saline mudflats; or (6) vegetated mudflats. The lake margin clastic facies includes deposits of: (1) birdfoot deltas; (2) stacked Gilbert-type deltas; (3) sheet deltas; (4) wave-reworked alluvial fans; or (5) wave-sorted sand sheets. Coal deposits are present in the lake margin clastic and the lacustrine facies of Carnian age (Late Triassic) only in basins of south-central Virginia and North and South Carolina. Eolian deposits are known only from the basins in Nova Scotia and Connecticut. Evaporites (and their pseudomorphs) occur mainly in the northern basins as deposits of saline soils and less commonly of saline lakes, and some evaporite and alkaline minerals present in the Mesozoic rocks may be a result of later diagenesis. These relationships suggest climatic variations across paleolatitudes, more humid to the south where coal beds are preserved, and more arid in the north where evaporites and eolian deposits are common. Fluctuations in paleoclimate that caused lake levels to rise and fall in hydrologically closed basins are preserved as lacustrine cycles of various scales, including major shifts in the Late Triassic from a wet Carnian to an arid Norian. In contrast, fluvial deposits were mainly formed in response to the tectonic evolution of the basins, but to some extent also reflect climatic changes. The Newark Supergroup illustrates the complexity of rift-basin sedimentation and the problems that may arise from using a single modern analog for sedimentary deposition spanning millions of years. It also shows that a tremendous wealth of depositional, climatic, and tectonic information is preserved in ancient rift-basin deposits which can be recovered if the depositional processes of modern rift-basin deposits are understood.

eastern North America↗
USGS1991

Change in the size of Walker Lake during the past 5000 years

In 1984, a 12-m sediment core (WLC84-8) was taken from the deepest part of Walker Lake. Samples of the core were analysed for diatoms, pollen, carbonate mineralogy, magnesium content, δ 18 O and δ 13 C values of the total inorganic fractin, δ 18 O and δ 13 C values of Limnocythere ceriotuberosa , δ 13 C values of the total organic fraction, grain size, and magnetic susceptibility. The data indicate that Walker Lake became shallow and probably desiccated between ⩾5300-4800 and 2700-2100 yr B.P.. Each of the organic and inorganic proxy indicators of lake size discussed in this paper was useful in determining the presence of the shallow-lake intervals. However, none of the indicators was useful in determining the cause of the shallow-lake intervals. Instead, the types of fish living in Walker Lake prior to 1940 were used to demonstrate that shallow-lake intervals resulted from diversion of the Walker River and not from climatic aridity. Major changes in mineralogy and magnesium content of carbonates and major changes in diatom populations with time were found to be a function of the chemical evolution of Walker Lake combined with changing lake size. The stable isotopes of oxygen and carbon were found to be good indicators of lake volume changes. A lake-level record for Walker Lake constructed from stable-isotope data was found to be similar to a lake-level record constructed using tufa and tree-stump data. Both records indicate relatively high lake levels between 4800−2700 yr B.P., at 1250 yr B.P., and within the last 300 yr. Substantial declines in lake level occurred ∼2000 and ∼1000 yr B.P.

California, Idaho, Nevada, Oregon↗
USGS1992

Setting and occurrence of Late Paleozoic radiolarians in the Sylvester allochthon, part of a proto-Pacific ocean floor terrane in the Canadian Cordillera

Late Paleozoic radiolarians have been used to establish th allochthon of the Slide Mountain terrane in British Columbia, and have thereby greatly clarified the geology and tectonic history of the terrane. As the Sylvester radiolarian fauna is limited, age assignments were based on a few distinctive and diagnostic robust forms. Radiolarians occur in cherts from a wide variety of different oceanic sequences that are structurally juxtaposed within the Sylvester allochthon. Like others in a suite of correlative terranes that lie along the length of the Cordillera, the Sylvester allochthon and the radiolarian bearing cherts in it derive from the telescoping together of slices from what was, in the late Paleozoic, a large area of the proto-Pacific ocean.

Canadian Cordillera↗
USGS1992

A mid-Permian chert event: Widespread deposition of biogenic siliceous sediments in coastal, island arc and oceanic basins

Radiolarian and conodont of Permian siliceous rocks from twenty-three areas in teh the circum-Pacific and Mediterranean regions reveal a widespread Permian Chert Event during the middle Leonardian to Wordian. Radiolarian- and (or) sponge spicule-rich siliceous sediments accumulated beneath high productivity zones in coastal, island arc and oceanic basins. Most of these deposits now crop out in fault-bounded accreted terranes. Biogenic siliceous sediments did not accumulate in terranes lying beneath infertile waters including the marine sequences in terranes of northern and central Alaska. The Permian Chert Event is coeval with major phosphorite deposition along the western margin of Pangea (Phosphoria Formation and related deposits). A well-known analogue for this event is middle Miocene deposition of biogenic siliceous sediments beneath high productivity zones in many parts of the Pacific and concurrent deposition of phosphatic as well as siliceous sediments in basins along the coast of California. Interrelated factors associated with both the Miocene and Permian depositional events include plate reorientations, small sea-level rises and cool polar waters.

western North America↗